1. Field of the Invention
The present invention relates to electrical systems for vehicles, and more particularly to a method and apparatus for achieving mechanical phase synchronization of multiple AC generators in an electrical power distribution system.
2. Description of the Related Art
Large vehicles such as aircrafts and ships require large amounts of energy to drive all onboard equipment. Increasing size and weight of energy producing systems, such as generators and motors, puts a limit on feasible size and complexity of large vehicles. “More electric” vehicles, which use electrical energy generators while eliminating bulky and vulnerable hydraulic systems, are being designed and produced. To satisfy the demand of electrical energy for “more electric” vehicles, large electrical generators are required. AC Variable Frequency generators provide an attractive solution, as they keep weight and cost down while increasing reliability. However, single AC Variable Frequency generators can not typically meet the power requirements of a vehicle with high electrical power demand, because an AC Variable Frequency generator capable of providing the required large electrical output would easily exceed technological, spatial, design, construction and installation limitations for both the generator and its prime mover.
In order to stay within spatial and technological limitations of generators while providing the large required electrical energy output, multiple smaller AC Variable Frequency generators can be combined to meet the required electrical energy output. However, the outputs of AC Variable Frequency generators that are not synchronized in phase and voltage cannot be combined, as current and voltage would be transferred between AC Variable Frequency generators without reaching the electrical distribution system of a vehicle. If the mismatch in frequency and voltage between AC Variable Frequency generators is large, their combined output can become unstable, leading to a shutdown or damage to the vehicle's electric system.
As a result, the use of multiple AC generators that are not synchronized requires one isolated bus for each AC generator to perform the necessary distribution of electrical generated power. As the number of prime movers and AC generators per prime mover increases for a vehicle application, the complexity of the electrical distribution system and of the electrical control system becomes extensive.
A few publications have studied systems that combine AC generators. One such technique is described in “Voltage Regulator Load Division Using Real and Reactive Generator Output Power Components to Control the Exciter”, Abdul Rashid, U.S. Pat. No. 5,077,485. With the method described in this work, a constant speed drive control and feedback system is used to combine generators to produce 400 Hz AC output. The constant speed drive control and feedback system adds significant complexity to the system. Moreover, this technique is not applicable to AC variable frequency generators installations.
Another technique is described in “Methods and Apparatus for Synchronizing Multiple Motor Driven Generators”, Robert Lee and Suresh Gupta, U.S. Pat. No. 4,575,671. In this publication, however, electric motors located between a prime mover and generators are de-energized for manual synchronization of generators using a slipping pole technique.
A disclosed embodiment of the application synchronizes multiple AC generators by utilizing a method and an apparatus for mechanical phase synchronization of multiple AC generators.